Wavelength division multiplexing (WDM) imposes new demands on laser sources employed in conjunction with optical fiber and free-space communications. One of these primary demands is the requirement for precision alignment of the source wavelength to one of the prescribed channels of a WDM system. Presently, distributed feedback (DFB) laser diodes are employed in such systems. Since the emission wavelength of a standard DFB laser diode varies with temperature at a typical rate of about 0.1 nm/.degree.C., their deployment in a WDM system requires precision control of the laser temperature, typically to better than .+-.0.1.degree. C. In order to provide laser sources that operate around room temperature within a tight wavelength tolerance, these types of lasers have to be individually sorted. In addition, screening of DFB laser sources is required to sort out those with low wavelength drift with aging.
WDM systems can be designed to run "open loop" where the channel alignment relies on the long term stability of the DFB wavelength with aging at constant temperature, or the laser emission wavelength can be controlled by an external passive reference filter. This is typically accomplished by monitoring the wavelength-dependent transmission of the filter, employing the laser temperature as a controlling parameter to match the laser output wavelength to the center wavelength of the filter. While such a feedback provides compensation and correction there are added costs to bear such as the external reference filter and required servo circuit controls in the feedback system.
A more simple approach is the use of a passive wavelength selection element in an optical fiber waveguide or a silica waveguide with a narrow band Bragg reflector embedded in an optical waveguide functioning as a wavelength filter. The optical waveguide is optically coupled to the laser chip output to provide optical feedback in an extended laser resonator that includes the laser chip rear facet at one end and the external reflector at the other end of the resonant cavity. The narrow band reflector locks the laser wavelength at the Bragg frequency of the fiber Bragg reflector and eliminates the need for additional external filters or servo circuits. Furthermore, the lower temperature sensitivity of the waveguide Bragg reflector center wavelength places tighter bounds on the overall wavelength drift with temperature compared to semiconductor DFB and DBR laser sources. An additional advantage is the ability to modulate the laser with low chirp in the extended, hybrid laser resonator. By directly modulating the current of the gain element, one can generate analog or digital data streams suitable for high speed optical data communication. In addition, by directly modulating the gain element of the waveguide DBR laser source, a train of stable pulses can be produced with tens of picoseconds pulse width having several to tens of milliwatts of peak power.
An antireflection (AR) coated semiconductor laser is thus employed as a gain element providing laser action through amplified stimulated emission while the optical fiber and the waveguide Bragg reflector function as the principal part of the optical cavity with very low loss.
Perhaps one of the largest concerns in the use of such hybrid extended laser cavities with a waveguide Bragg grating is the stability of longitudinal mode selection in operation of the laser source. The ability to produce waveguide DBR lasers with tightly controlled operating wavelengths is important for WDM systems, particularly dense WDM systems requiring closely spaced fixed wavelengths. However, the waveguide DBR by itself will not be stable in single longitudinal mode operation because longitudinal mode transition can occur with changes in environmental temperature or current pumping. A primary source of this instability stems from the difference in the dependence of refractive index in the different components making up the resonant cavity. What is needed is some way of stabilizing the waveguide DBR taking into consideration these foregoing mentioned factors of temperature changes and refractive index differences.
An object of this invention is to provide a stabilized waveguide DBR laser source.
Another object of this invention is to provide a directly modulated waveguide DBR laser capable of operating at high modulation rates in excess of 2.0 Gb/s.
It is another object of this invention to provide a waveguide DBR laser that can be assembled to ensure reliable operation with aging and environmental temperature changes.